I remember reading that there are vets who can spay while they are in heat but it costs more than normal because her stuff is enlarged or swollen. If you can handle waiting til she is out of heat and take her in then, that might be better. But that is just my two cents.
I called for an appointment for spaying the day after my cat went into heat. They made an appointment, for the following week. They said, you can't really know when she's going into heat again, so best to just have it done.
Bailey was out of heat by the time the appointment came around. But they can do the spay when the cat is still in heat.